1. Field of the Invention
The present invention relates to a method of fabricating a stainless martensitic steel, comprising a step of electroslag remelting of an ingot of said steel then a step of cooling said ingot, then at least one austenitic thermal cycle consisting in heating said ingot above its austenitic temperature.
In the present invention, unless otherwise stated, the composition percentages are percentages by weight.
2. Description of the Related Art
A stainless martensitic steel is a steel with a chromium content of more than 10.5% and of a structure that is essentially martensitic.
It is important for the fatigue behavior of such a steel to be as good as possible so that the service life of parts produced from such a steel is maximized.
To this end, it is sought to improve the inclusion characteristics of the steel, i.e. to reduce the quantity of undesirable inclusions (certain alloy, oxide, carbide, and intermetallic compound phases) present in the steel. Such inclusions act as crack initiation sites that, under cyclic loading, result in premature failure of the steel.
Experimentally, a large dispersion is observed in the results of fatigue tests carried out on test specimens of that steel, i.e. for each level of fatigue loading under imposed deformation, the service life (corresponding to the number of cycles resulting in breaking of a fatigue specimen in that steel) varies over a wide range. Inclusions are responsible for the minimum values, in the statistical sense, for the fatigue service life of the steel (low values of the range).
In order to reduce that dispersion in fatigue behavior, i.e. in order to raise those low values, and also to enhance the mean fatigue behavior value, it is necessary to improve the inclusion characteristics of the steel. The electroslag remelting technique, ESR, is known. In that technique, the steel ingot is placed in a crucible into which a slag (mixture of minerals, for example lime, fluorides, magnesia, alumina, calcite) is poured such that the lower end of the ingot is immersed in the slag. Next, an electric current is passed through the ingot, which acts as an electrode. That current is sufficiently high to heat and liquefy the slag and to heat the lower end of the steel electrode. The lower end of that electrode is in contact with the slag, and so it melts and passes through the slag in the form of fine droplets, and then solidifies below the layer of slag, which floats, to form a new ingot that therefore grows gradually. The slag acts, inter alia, as a filter that extracts the inclusions from the steel droplets, such that the steel of that new ingot located below the layer of slag contains fewer inclusions than the initial ingot (electrode). That operation is carried out at atmospheric pressure and in air.
Although the ESR technique can reduce the dispersion in the fatigue behavior of stainless martensitic steels by eliminating inclusions, that dispersion is still too large in terms of the service life of the parts.
Non-destructive testing using ultrasound carried out by the inventors has shown that said steels include practically no known hydrogen defects (flakes).
The dispersion of the fatigue behavior results, specifically the low end values of the range of results, is thus due to another undesirable mechanism of premature initiation of cracks in the steel, which results in premature fatigue breaking.